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I've just finished reading Marcus Chown's new book We Need to Talk about Kelvin (in the shops in a couple of weeks time), which starts from everyday observations to probe different aspects of basic physics and cosmology.

The final chapter is a little different, in that the concept being probed doesn't have a good scientific theory to cover it - it's 'why aren't the aliens here?'

The point is that it's a huge universe that should contain many planets capable of supporting life. So you'd expect there to be lots of intelligent life out there. And dismissing most UFO sightings etc. as the errors and wishful thinking that they indubitably are, we are shockingly short of alien observations. Why aren't they on the street corners? Why don't we see their probes and receive their messages?

In practice, the 'street corners' question is easy to answer. Aliens are just as limited by the speed of light as we are - and it is a very big universe. So the only sensible way to explore is using self-replicating probes - but they should be here by now.

There seem to be three serious possibilities for this situation. One is that aliens just don't want to come here. Maybe we smell (in the cosmic sense). Or we're just too insignificant. The second is that they are here, but they're too clever to let us know. And the third as that we are the only intelligent life - at least in the Milky Way galaxy.

On the whole, scientists don't like special cases like 'there's only us' - but the circumstances to produce a 'Goldilocks planet' - one that's just right for intelligent life as we know it to form on - are quite specific, and it's possible to envisage a number of scenarios where we are the only ones.

Whichever possibility is right, it's worth remembering that the concept of alien life isn't just something for the science fiction shows. It can tell us quite a lot about our place in the universe.

Hi BrianWhilst viewing Most Glamorous’ website, I noticed that you’d had some success via her twitter comments, so I thought I’d take a look – before I investigate the mysterious blog that has appeared on her latest post. Congratulations on your award! And I hope you receive even more recognition in the future!I also noticed that you had posted something about aliens – one of my favourite subjects. My latest work contains aliens on earth, so I read on with interest. Solvey has read part of it, and his enthusiasm for it has caused me to do a lot more research than I intended. I want to do it justice!I do find it hard to believe that aliens might not know of our existence, given that the concrete evidence of the existence of this planet, of which we are a part, would suggest that we could not be invisible to them. To view a blade of grass, or a rock, or the planet as a whole which combines at least these two, suggests that we could not be excluded involuntarily. We may not be aware of them, however, if they had the means to make us unaware of them. This would suggest a greater intelligence and technology on their part – unless this just happened to be a natural phenomenon peculiar to them. If they came here after we sprouted up, then that would be a natural assumption. If they came here before us, and live here either by necessity or by choice, then we may never get to see them.Oh, there are a million things I would say concerning aliens, or any kind of life. There are constants to take into account, though: Light, Solidified matter (planets?), Gravity, Life itself, Intelligence. Elements that help create life are not so important – I think that life will form wherever it must. I once suggested a theory of everything L=I (Life =Infinity), where life would exist no matter the conditions, or lack of them. In other words, we cannot state a random theory as we do not have a full grasp of all of the requirements for life to hand. There may even only be one prerequisite for life, and that may be Light itself. The ‘Solar Unite’ from a film of much unintentional hilarity. Darkness is relative to the amount of light, so may have little bearing on life’s creation and may not even exist. Nuts, aren’t I lol!!!On another note, what would you say to the suggestion that the laws of physics never break down? That even a Big Bang is controlled by physical parameters and knock-on effects that control the events up to, during, and after such an event. If laws of physics broke down at any moment, the whole universe would collapse. Such a thing may not be possible, or allowed – by a higher intelligence in control of it!Endless! lol

Hi Esruel,You're right, our planet is a good sign of our existence, if you happen to look at it, but given the vastness of the universe, we're easy to miss.

As for the laws of physics breaking down, it's certainly possible that they have varied over time, or may be different in different parts of the universe - see my latest book Before the Big Bang for more on this!

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